42 | New Scientist | 18 April 2020
it probably wasn’t the first alligator to be eaten
by giant isopods. Conservation efforts mean
there are now close to a million American
alligators in the southern US – the specimens
LUMCON used for this and other drops were
obtained from a culling programme. Storms
can sweep dead reptiles offshore. Elsewhere,
saltwater crocodiles, caimans and other large
reptiles live along coastlines and could also
end up sinking, without the help of scientists.
The aim of the alligator study, led by Craig
McClain, executive director at LUMCON, was to
trace the effects of this reptilian carbon in the
abyss. Alligators, crocodiles and caimans are
the closest living analogues of the enormous
marine reptiles such as ichthyosaurs and
plesiosaurs that dominated the oceans in
the Jurassic and Triassic eras between 250 and
66 million years ago. Their bodies would have
made a significant contribution to the deep-
sea carbon budget. McClain wanted to find
out which scavengers are lying in wait in the
deep ocean for a modern-day reptilian meal.
By the time the submersible returned to the
scene, the dead alligator had lain in the abyss
for 50 days. It had been stripped to bare bones.
No one had been there to watch the feast take
place but, based on similar studies of dead
whales, we can guess how events unfolded.Food oasis
Sunken whales form food-rich islands in
the deep that last for decades. First, mobile
scavengers, such as the eel-shaped, slime-
producing hagfish and bristle-covered
polychaete worms, strip away the soft tissues.
Opportunists like crabs and snails congregate
around the carcass to feed off scraps dropped
by the scavengers. If a dead whale is in the deep
for long enough, bacteria begin to break down
its bones anaerobically, releasing sulphur and
methane. These “chemosynthetic” microbes
grow in thick mats, turning the whale bones
white, yellow and pink, attracting shrimp,
crabs and snails to graze on them.
When McClain and the team returned to
the fallen alligator, it was probably too soon
for sulphurous, chemosynthetic stages to
have developed. But the carcass did harbour
something else commonly known from whale
falls. The alligator bones had a “furry, red shag
carpet-look to them”, says McClain. This red
fuzz turned out to be a type of worm called
Osedax (meaning bone-eating in Latin).
Sometimes known as zombie worms, Osedax
look more like plants than worms, with roots
and pink, flowery gills. They have no mouth
and no stomach, instead absorbing anddigesting collagen from the bones through
their roots, with the help of symbiotic
microbes living inside them.
First discovered on a whale fall in 2002,
dozens more such worms have since been
found on a variety of sunken bones. But two
of the types found on the alligator were
previously unknown species.
Some researchers initially thought Osedax
may specialise in eating whale bones, but it
was soon established that they will colonise
any bones they find. They do, however, need
a large vertebrate, like a whale, whose skeleton
will remain on the deep seabed for a long time,
not chewed up and eaten whole by a predator.
This raises the question of which came first,
the whales or bone-eating worms.
To find out, we can look at the genes of
different Osedax species, comparing them with
other types of worm to find out when they first
evolved. However, the results of such genetic
studies can give conflicting results depending
on how they are processed. One of the genetic
clocks used to trace the evolutionary timescale
indicated that the zombie worms originated
45 million years ago, shortly after the ancestors
of whales took to the oceans. But another
genetic clock put the origin of Osedax back in
the Cretaceous era, which began 145 million
years ago. If that clock tells the correct
evolutionary time, whose bones were those
ancient worms eating?
An answer came in 2015, when a team at the
University of Plymouth took a fossilised bone“ What scavengers
are lying in
wait in the
deep ocean
for a reptilian
meal?”
The bone-eating
Osedax is more like
a plant than a worm
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